1. Field of the Invention
The present invention relates to a magnetic disk apparatus for use as data storage means in, for example, a personal computer.
2. Description of the Related Art
Recently, magnetic disk apparatuses have been developing a tendency to smaller size and lighter weight, and are expected to be positively improved in assembly accuracy. In a small-sized magnetic disk apparatus, in particular, a casing and a top cover are manufactured using the same material, e.g., aluminum, so that bad influences of thermal strain of these members are reduced.
In general, magnetic disk apparatuses comprise a voice coil motor (hereinafter referred to as VCM) for moving a magnetic head in the radial direction of a magnetic disk. The VCM includes a voice coil and a magnet, which can move relatively to each other, and a yoke. This magnet generates a magnetic flux directed to the outside of the casing, as well as one directed to the coil. The greater part of the outward magnetic flux passes through the yoke and the top cover bound for the opposite magnetic pole, while the remaining part leaks out through the top cover. It is advisable, therefore, to provide the magnetic disk apparatuses with a magnetic shielding mechanism for preventing the magnetic flux from the magnet from leaking to the outside of the casing. In a relatively large-sized magnetic disk apparatus, the casing has a generous inside space, so that the magnetic shielding mechanism can be easily set in the casing.
In a small-sized magnetic disk apparatus, however, the inside space of the casing is so limited that it is difficult to provide the mechanism for preventing leakage of the magnetic flux. In the aforesaid case where both the casing and the top cover are formed from aluminum, in order to reduce the weight and prevent thermal strain, in particular, the magnetic flux from the magnet leaks out at once. The leakage of the magnetic flux exerts a bad influence on other equipment located close to the magnetic disk apparatus. In a production line for magnetic disk apparatuses, for example, leakage of the magnetic flux from one apparatus may possibly destroy data recorded on magnetic disks in adjacent apparatuses.